Electrochemichromic sunroof

ABSTRACT

The specification discloses electrochemichromic solutions and devices based on the use of solvents comprising at least about 25% 3-hydroxypropionitrile, 3,3,&#39;-oxydipropionitrile, 2-acetylbutyrolactone, 2-methylglutaronitrile, 3-methylsulfolane and mixtures thereof. The specification also discloses vacuum backfilling techniques for filling electrochemichromic cells and enhanced UV stability through solvent self-screening.

This is a continuation application of Ser. No. 07/878,176, filed May 4,1992, now U.S. Pat. No. 5,340,503, which is a division of applicationSer. No. 07/443,113, filed Nov. 29, 1989, which issued as U.S. Pat. No.5,140,455.

BACKGROUND OF THE INVENTION

The present invention relates to electrochemichromic solutions anddevices based thereon. Such solutions are well-known and are designed toeither color or clear depending on desired application, under theinfluence of applied voltage.

Such devices have been suggested for use as rearview mirrors inautomobiles such that in night driving conditions, application of avoltage would darken a solution contained in a cell incorporated intothe mirror (U.S. Pat. No. 3,280,701, Oct. 25, 1966). Similarly, it hasbeen suggested that windows incorporating such cells could be darkenedto block out sunlight, and then allowed to lighten again at night.Electrochemichromic cells have been used as display devices and havebeen suggested for use as antidazzle and fog-penetrating devices inconjunction with motor vehicle headlamps (British Patent Specification328017, May 15, 1930).

U.S. Pat. No. 4,090,782 to Bredfeldt et al., U.S. Pat. No. 4,752,119 toUeno et al. (June 1988), Chemical Abstract 86:196871c, 72-Electro.Chemistry, Vol. 86, 1977, I. V. Shelepin et al. in Electrokhimya, 13(3),404-408 (March 1977), O. A. Ushakov et al., Electrokhimya, 14(2) 319-322(February 1978), U.S.S.R. Patent 566863 to Shelepin (August 1977), U.S.Pat. No. 3,451,741 to Manos, European Patent Publication 240,226published Oct. 7, 1987 to Byker, U.S. Pat. No. 3,806,.229 to Schoot etal., U.S. Pat. No. 4,093,358 to Shattuck et al., European PatentPublication 0012419 published Jun. 25, 1980 to Shattuck and U.S. Pat.No. 4,139,276 to Clecak et al. all disclose electrochemichromicsolutions of anodic and cathodic electrochromically coloring componentswhich provide self-erasing, high color contrast, single compartmentcells. Such anodic and cathodic coloring components comprise redoxcouples selected to exhibit the following reaction: ##STR1## The redoxcouple is selected such that the equilibrium position of the mixturethereof lies completely to the left of the equation. At rest potential,the anodically coloring reductant species RED₁, and the cathodicallycoloring oxidant species OX₂ are colorless. To cause a color change,voltage is applied and the normally colorless RED₁ is anodicallyoxidized to its colored antipode OX₁, while, simultaneously, OX₂ iscathodically reduced to its colored antipode, RED₂. Thesecathodic/anodic reactions occur preferentially at the electrodes which,in practical devices, are typically transparent conductive electrodes.Within the bulk of the solution, the redox potentials are such that whenRED₂ and OX₁ come together, they revert to their lower energy form.

This means the applied potential need only suffice to drive the abovereaction to the right. On removing the potential, the system reverts toits low energy state and the cell spontaneously self-erases.

Such redox pairs are placed in solution in an inert solvent. Typically,an electrolyte is also added. This solution is then placed into arelatively thin cell, between two conductive surfaces. In mostapplications, at least one of the conductive surfaces comprises a verythin layer of a transparent conductor such as indium tin oxide (ITO),doped tin oxide or doped zinc oxide deposited on a glass substrate sothat the cell is transparent from at least one side. If the device is tobe used in a mirror, the second surface is typically defined by arelatively thin layer of transparent conductor such as indium tin oxide,doped tin oxide or doped zinc oxide deposited on another glasssubstrate, which is silvered or aluminized or otherwise reflector coatedon its opposite side. In the case of solar control windows, the secondglass substrate would of course not be silvered on its opposite side sothat when the redox pair is colorless, the window would be entirelytransparent.

A wide variety of cathodically coloring species, anodically coloringspecies, inert current carrying electrolytes and solvent systems aredescribed in prior art. However, combinations of these suitable to meetthe performance required for outdoor weathering, particularly foroutdoor weathering of automobile rearview mirrors and automobile andarchitectural windows, have hitherto not been revealed. Nor havecombinations been revealed that, in conjunction with possessing inherentUV stability, meet the temperature extremes required in commercialautomotive and architectural applications. Nor have combinations beenrevealed that meet the UV resilience and temperature extremes requiredin automotive and architectural applications and that simultaneouslyhave sufficiently low vapor pressures to facilitate use of a vacuumbackfill technique to fill thin cells where the interpane spacing isvery small. With higher vapor pressures, undesirable voids are left withthe solution in the vacuum backfilled cell.

Vacuum backfilling has been used to fill liquid crystal displays. Liquidcrystal displays are typically much smaller than the large areas oftypical electrochemichromic devices such as mirrors and windows. Liquidcrystal materials have inherently high viscosity and low vapor pressure.To fill with liquid crystal using the vacuum backfill technique,elevated temperatures are typically used so that the liquid crystalviscosity is sufficiently low that the material flows into and fills thecavity. Because of their inherent low vapor pressure even at elevatedtemperatures, voids are not a significant problem during backfillingwith liquid crystals. The same is not true for many electrochemichromicsolvents cited in the prior art.

Many of the organic solvents proposed in the prior art as solvents forelectrochemichromic compounds have disadvantages when chosen for UVresilient devices. This is because commonly suggested solvents, such asacetonitrile, propylene carbonate, gamma-butyrolactone, methyl ethylketone, dimethylformamide and the like, are highly transmissive to UVradiation. Incoming UV radiation that is admitted by the ITO-coatedglass substrate is unattenuated by the solvent and thus is capable ofphotolyzing or otherwise degrading any UV vulnerable solute in solutionin that solvent.

Addition of UV stabilizers such as benzotriazoles, benzophenones, orhindered amine complexes, as known in prior art, can help increasesolution stability to UV radiation, but there are limitations anddisadvantages to addition of UV stabilizers. Because they are held insolutions of low to moderate viscosity, both the UV stabilizer and theelectrochemichromic solutes are free to randomly move about in thesolution. Thus, an incoming photon of UV radiation may impinge and thusdegrade an electrochemichromic solute species rather than be absorbed bya UV absorber in solution. Also, solubility within the selected solventplaces limits on the amount of UV stabilizer that can be added.

Solute solubility is also a factor in connection with the choice ofsolvents for electrochemichromic components. High solubility ispreferred for the anodic and cathodic species as well as forelectrolytes which are usually added to such solutions, Suchelectrolytes enhance cell performance and must be soluble in thesolvent.

Yet another problem encountered in electrochemichromic devices relatesto current leakage. When the electrochemichromic cell is colored by theapplication of voltage, the colored species OX₁ and RED₂ continuallywant to recombine and return to their equilibrium, colorless condition.The rate of recombination of the colored species OX₁ and RED₂ within thebulk of the solution is directly proportional to their diffusioncoefficient in the solvent used. In order to compensate for the tendencyof the colored species to recombine and go to the colorless equilibriumstate, current must continually leak into the electrochemichromicsolution via the conductive electrodes that typically sandwich saidsolution.

Because current must flow across the conductive surface of thetransparent conductor used on at least one of the substrates thatsandwich the electrochemichromic cell, and because these transparentconductors have finite sheet resistance, applied potential will behighest adjacent to the bus bar connector typically located at an edgeperimeter and will be lowest near the center of the device as currentpasses across the conductive glass surface to color remote regions.Thus, if the leakage current is high and/or the sheet resistance of thetransparent conductor is high, the potential drop that ensues across thetransparent conductor itself results in a lower potential being appliedto remote regions. Coloration is therefore nonuniform with the edgeregions nearest the bus bar coloring deepest and the central regionscoloring lightest. Such nonuniformity in coloration is commerciallyundesirable. For a given transparent conductor sheet resistance, thelower the leakage current the more uniform the coloration. This is animportant advantage; otherwise, a thicker and hence more costly and lesstransparent conductive coating would be needed to reduce the sheetresistance to accommodate the higher leakage currents seen with solventssuggested in the prior art.

Yet another disadvantage of higher leakage currents is their impositionof a drain on battery-power sources in some instances. If anelectrochemichromic device were used in a sunroof, for example, it wouldbe desirable to have the sunroof colored dark while the car is parked ina parking lot. If the current leakage is too great, the operator couldfind that the car battery has been drained as a result of current beingdrawn by the colored sunroof.

One further problem which plagues electrochemichromic devices is"segregation". When first bleached after being held for a prolongedperiod in the colored state, bands of color are seen adjacent to the busbar connectors to the transparent conductive electrodes that sandwichthe electrochemichromic solution. In electrochemichromic solutionsrevealed in prior art, various methods must be used to reducesegregation. These include thickening the electrochemichromic solution,use of low concentrations of electrochemichromically active species, anduse of high concentrations of current-carrying electrolyte. The additionof thickeners will also reduce leakage current. One problem with addingthickeners is that the solution can become so viscous that vacuumbackfilling a thin electrochemichromic cell becomes commerciallyunfeasible.

As a result of these drawbacks, electrochemichromic solutions anddevices based thereon have not achieved the degree of commercial successwhich they potentially could achieve.

SUMMARY OF THE INVENTION

The present invention comprises an electrochemichromic solution anddevices, based on the use of a solvent comprising at least about 25% byvolume of a solvent selected from the group consisting of3-hydroxypropionitrile (HPN), 3,3'-oxydipropionitrile (ODPN),2-acetylbutyrolactone (ABL), 2-methylglutaronitrile (MGNT),3-methylsulfolane (MS) and mixtures thereof. The foregoing solutionsexhibit unexpectedly lower current leakage and generally superior UVcharacteristics, either alone or with UV stabilizers added. Segregationis minimized without excessive solution viscosity increase.

Segregation is minimized without the necessity of adding thickenerswhich cause excessive viscosity increase. Also, excellent performance inboth cycle behavior and in segregation performance can be achieved usinglow concentration of added inert electrolyte. These and other objects,advantages and features of the invention will be more fully understoodand appreciated by reference to the written specification and appendeddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a solar spectrum in the ultraviolet region as determined inTucson, Ariz.;

FIG. 2 is a cross-sectional view of an electrochemichromic cell;

FIG. 3 is a graph of the percent transmittance of solar radition atvarious wavelengths through a piece of glass coated with a half waveindium tin oxide coating;

FIG. 4 is the solar spectrum passed by a piece of glass coated with halfwave indium tin oxide;

FIG. 5 is a graph of the percent transmittance of solar radiation atvarious wavelengths for 0.0002 molar acetonitrile solutions of variouscathodic compounds typically used in electrochemichromic cells;

FIG. 6 is a graph of the percent transmittance of solar radiation atvarious wavelengths by 0.0002 molar acetonitrile solutions of variousanodic compounds used in electrochemichromic cells;

FIG. 7 is a graph of the percent transmittance of solar radiation atvarious wavelengths by the solvent ABL; and

FIG. 8 is a graph of the percent transmittance of solar radiation atvarious wavelengths by prior art solvents for electrochemichromicsolutions.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Introduction:

The electrochemichromic solutions of the preferred embodiment canutilize conventional or equivalent redox systems such as the viologenscombined with phenazines, diamines or benzidines, dissolved in a solventcomprising at least 25% by volume of one or more of the solvents of thepresent invention: 3-hydroxypropionitrile (HPN), 3,3'-oxydipropionitrile(ODPN), 3-methylsulfolane (MS), 2-methylglutaronitrile (MGNT) and2-acetylbutyrolactone (ABL). Electrolytes may optionally be used and arepreferably used.

Viologens are preferred cathodic materials for the redox pair.Methylviologen, ethylviologen, benzylviologen and heptylviologen are allsatisfactory, with a 0.025 molar solution of methylviologen beingpreferred. Higher concentrations up to the solubility limits are alsooperable. In the structural formulas set forth below, X⁻ represents theanion of the viologen salt. Various anions are disclosed in theliterature, though we have discovered that the most preferred anion ishexafluorophosphate (PF₆ ⁻) because it surprisingly enhances viologensolubility. This preferred embodiment will be the subject of a copendingU.S. patent application to be entitled ELECTROCHEMICHROMIC VIOLOGENS.##STR2## Hexafluorophosphate counter ion is listed below with otheracceptable, though less preferred, counter ions for use on theviologens:

    ______________________________________                                        Tetrafluoroborate      BF.sub.4.sup.-                                         Perchlorate            ClO.sub.4.sup.-                                        Trifluoromethane sulfonate                                                                           CF.sub.3 SO.sub.3.sup.-                                Hexafluorophosphate    PF.sub.6.sup.-                                         ______________________________________                                    

The preferred anodic coloring materials are set forth below: ##STR3##Most preferred is a 0.025 molar solution of5,10-dihydro-5,10-dimethylphenazine (DMPA).

Numerous electrolytes can be used in the present invention. One which isoften suggested for electrochemichromic cells and which is acceptable inaccordance with the preferred embodiment of the invention is atetrabutylammonium hexafluorophosphate. We prefer a 0.025 molarsolution.

UV stabilizers such as Uvinul™ 400 at approximately 5% weight by volumecan also be used in the solutions of the present invention. As explainedbelow, such UV stabilizers are more preferably used in connection withsome of the solvents of the present invention than others.

The best mode electrochemichromic solution contemplated for practicingthe invention comprises one of the solvents of this invention,containing 0.025 molar methylviologen hexafluorophosphate, 0.025 molartetrabutylammonium hexafluorophosphate, and 0.025 molar5,10-dihydro-5,10-dimethylphenazine (DMPA).

FIG. 2 illustrates a typical electrochemichromic cell 1 into whichsolutions of the present invention are typically filled. Cell 1comprises a pair of glass plates 10 and 11 each coated on its inwardlyfacing surface with a half wave indium tin oxide (ITO) coating 12 ofabout 15 ohms/square sheet resistance. Plates 10 and 11 are separated byperipheral seal 13 so that the interior of the cell has a thickness of150 microns. Cell 1 is sealed at its perimeter by peripheral seal 13.Seal 13 comprises an epoxy material, to which 150 micron diameterspacers are added, and silk-screened to a thickness of about 150microns. Glass beads are used as spacers. As shown, cell 1 is intendedto be used as a mirror, and thus the rear surface of glass plate 11 iscoated with a silver reflector layer 14. If the device were used as awindow, layer 14 would be deleted. The conductive indium tin oxidelayers 12 are connected to electrical terminals 15 and 16 so that a ovoltage can be established across a solution located between plates 10and 11 in cell 1.

To vacuum backfill cell 1, a small gap is introduced into seal 13 atsome extremity corner. This acts as a fill hole. Solution can be filledthrough this hole once inside the cell, the solution is contained byseal 13 between glass substrates 10 and 11. It is desirable to use asmall fill hole so that the entrance orifice is small. Otherwise, it isdifficult to seal the fill hole once the cell cavity is full such thatno leaks occur through the fill hole. But since the fill hole is small,less than 1 mm×1 mm×150 microns typically, it is difficult to fill thecell cavity using a hypodermic needle or the like. Also, since there isonly one fill hole, back pressure would impede complete filling throughone fill hole anyway. Thus a means is needed to fill such a cell cavitythat overcomes the above problems. Vacuum backfilling is such a means.

In the vacuum backfill technique, the empty cell is placed in a vacuumchamber along with a container (typically a dish or small cup) of theelectrochemichromic fluid intended to be filled through the single fillhole into the cell cavity. The chamber is evacuated to a high vacuum, 1mm Hg or better. Means are then used to lower the fill hole just underthe surface of the electrochemichromic fluid. The chamber is now ventedto atmospheric pressure (typically using nitrogen or similar inert gas).Atmospheric pressure forces the fluid into the cell cavity and so fillsit. However, how completely it fills is a function both of the vacuumpressure upon evacuation P_(V) and the atmospheric pressure P_(A) towhich the chamber is vented during venting.

Although a vacuum pump can evacuate vacuum chamber to 10⁻⁶ mm Hg orbetter, the vapor pressure of the solvent limits how high a vacuum canbe achieved. This is because the vacuum pump reduces the vacuum pressuredown to the vapor pressure (at the temperature of the chamber) of thefluid used. Once the vacuum pressure equals the vapor pressure, vacuumpressure will go no lower until all the fluids have evaporated. Thus thechoice of solvent, through its vapor pressure, dictates how large abubble will remain after backfilling a given cell volume. As the devicearea increases such as might be encountered in window devices, theproblem gets worse and, unless a sufficiently low vapor pressure solventis chosen, or unless means such as cooling the fluid and chamber (toreduce vapor pressure) or overpressuring during backfill (to force morefluid in) are employed, a cosmetically unacceptable bubble will be leftwithin the electrochemichromic cell. While a small bubble of about 1 mmdiameter may dissolve over time, a larger bubble will not completelydisappear. Further, if the viscosity of the fluid to be filled is veryhigh, then it may be difficult to fill at room temperature. If higherfilling temperatures are used, the residual bubble may be larger as thevapor pressure increases with temperature. Simple physics teaches that:

    P.sub.A V.sub.A =P.sub.V V.sub.V                           (1)

where

P_(A) =pressure to which the chamber is finally vented.

V_(A) =volume of gas trapped in the cell after completely filling thecell.

P_(V) =vacuum pressure in the chamber after evacuation and prior tofilling.

V_(V) =volume of the empty cavity, i.e., cell volume.

Since undissolved gas trapped in the cell after incomplete filling willusually form a bubble, then V_(A) can be written as:

    V.sub.A =π/4 d.sup.2 t                                  (2)

where

d is the bubble diameter; and

t is the cell cavity thickness.

Also, P_(A) is usually 760 mm Hg although it is important to stress thatthe-chamber can be overpressured to several atmospheres or more afterfilling if it is desired to fill more completely. However, in the casewhere P_(A) =760 mm Hg and where V_(V) =A×t where A is the cell area andt is the interpane thickness, we have:

    P.sub.V ·A·t=760·π/4·d.sup.2 ·t                                               (3)

which reduces to

    P.sub.V =5.969 d.sup.2 /A                                  (4)

where d is in mm and A is in cm²

Likewise ##EQU1## Equation (4) expresses the relationship between theresidual gas bubble diameter d (in mm) and the cell area (in cm²) to thepressure in the chamber, P_(V), prior to venting to atmosphere and thusbackfilling.

Note that if two solvents or more are mixed together to form an idealsolution, the vapor pressure of the solution is simply the sum of thevapor pressures of each component. The solvents taught in this inventionhave very low vapor pressures, in some cases exceptional so that theyare excellent choices for use as solvent components inelectrochemichromic solutions intended to be vacuum backfilled. This isparticularly important when large area devices such as 1 m² windowswhere the volume of cell cavity can be as large as 150 cc orthereabouts. By contrast, many of the prior art solvents, such asacetonitrile, methylethylketone, and dimethylformamide are unsuitablechoices, even for use as components in solvent mixtures. Also, note thatthe solutions used as electrochemichromic fluids are sufficiently dilutefor the various solutes (anodic/cathodic compounds, electrolyte, etc.)not to significantly depress vapor pressures.

Lower boiling point solvents such as acetonitrile, dimethylformamide andmethylethylketone tend to have relatively high vapor pressures at roomtemperature. Thus, the higher boiling point solvents of the presentinvention, which tend to have lower vapor pressures at room temperature,are significantly more suitable for the vacuum backfilling technique ofthe present invention. They tend to leave smaller bubbles in the filledcell.

2-acetylbutyrolactone is most desirable, since its boiling point is sohigh that it is measurable at atmospheric pressure only with greatdifficulty. Even at 5 mm Hg (0.0066 Arm), it boils at 107° C. Thus, ABLvapor pressure is so low that it is most preferable of the solvents ofthis invention for purposes of vacuum backfilling.

The Experimental Data, Tables 1, 2 And 3:

Table 1 compares the solvents of the present invention to threeconventionally suggested prior art electrochemichromic solvents:propylene carbonate, gamma butyrolactone and dimethylformamide. Thefirst and second columns report boiling point and freezing point for thevarious solvents, including those of the present invention. The thirdcolumn indicates the appearance of electrochemichromic solutions at zeroapplied potential made in accordance with the present invention in anelectrochemichromic cell.

Electrochemichromic cell 1 (FIG. 2) was used for the data contained inthe third column of Table 1 and the data in Table 2. Cell area was about110 cm² and thickness was about 150 microns. Sheet resistance of the ITOtransparent conductors used in the cell was 15 ohms per square. ForTable 2, the cells were powered to 1 volt. Each cell was filled with asolution of the indicated solvent or solvent combination, containing0.025 molar methylviologen perchlorate, 0.025 molar 5,10-dihydro5,10-dimethylphenazine and 0.025 molar tetraethylammonium perchlorateunless otherwise indicated in the Table. Conventional techniques wereused to ensure the solutions were oxygen free and were anhydrous. Table2 compares electrochemichromic solutions which are identical in allrespects, except that different solvents are used. Propylene carbonate(PC), gammabutyrolactone (GBL), dimethylformamide (DMF) and acetonitrile(AN), conventional solvents, are compared to 3-hydroxypropionitrile(HPN), 3,3'-oxydipropionitrile (ODPN), 3-methylsulfolane (MS),2-methylglutaronitrile (MGNT) and 2-acetylbutyrolactone (ABL).

The first four data columns of Table 2 report reflectivity data.Reflectivity is measured in a conventional manner using standardilluminant A and a photodetector that reproduces the eye's photopicresponse and is expressed as a percentage of incident light which isreflected by the mirror. The first data column discloses the highpercentage reflectivity as measured when the electrochemichromicsolution is at zero potential and thus is colorless. The second columnmeasures the low percent reflectivity, which is determined when theelectrochemichromic solution is colored at 1 volt applied potential.

The third column measures the time in seconds that it takes for thesolution to color from 70% reflectivity to 20% reflectivity. The fourthcolumn indicates in seconds the time it takes for the solution to bleachfrom 10% reflectivity to 60% reflectivity. The fifth column of Table 2measures current leakage for the fully colored solution presented inamperes per square meter.

Table 3 discloses the solubility of various UV stabilizers in thesolvents of the present invention as compared to a prior art solvent,propylene carbonate. In most instances, the UV stabilizers aresubstantially more soluble in the solvents of the present invention, theonly exception being that Tinuvin p™ is only marginally soluble in anyof the solvents and Uvinul N-539™ is mostly immiscible with HPN.

                  TABLE 1                                                         ______________________________________                                                      Boiling    Freezing  Color                                      Solvent       Point      Point     In Cell                                    ______________________________________                                        Propylene carbonate                                                                         240° C.                                                                           -55° C.                                                                          Clear and                                   ##STR4##                          Colorless                                  (Prior art)                                                                   γ-Butyrolactone                                                                       205° C.                                                                           -45° C.                                                                          Clear and                                   ##STR5##                          Colorless                                  (Prior art)                                                                   Dimethylformamide                                                                           153° C.                                                                           -61° C.                                                                          Clear and                                   ##STR6##                          Colorless                                  (DMF)                                                                         (Prior art)                                                                   3-Hydroxypropionitrile                                                                      228° C.                                                                           -46° C.                                                                          Clear and                                  or 2-cyanoethanol                  Colorless                                  or hydracrylonitrile                                                          HOCH.sub.2 CH.sub.2 CN (HPN)                                                  3,3'-oxydipropionitrile                                                                     188° C./                                                                          -26° C.                                                                          Clear and                                  O(CH.sub.2 CH.sub.2 CN).sub.2                                                               16 mm Hg             Colorless                                  (ODPN)                                                                        2-acetylbutyrolactone                                                                       107° C./                                                                          <-33° C.                                                                         Clear and                                  (ABL)         5 mm Hg              Colorless                                   ##STR7##                                                                     (α-acetobutyrolactone)       Clear and                                                                     Colorless                                  2-methylglutaro-                                                                            125-130° C./                                                                      <-33° C.                                                                         Clear and                                  nitrile (MGNT)                                                                              10 mm Hg             Colorless                                   ##STR8##                                                                     3-methylsulfolane                                                                           276° C.                                                                           -10° C.                                                                          Clear and                                  (MS)                               Colorless                                   ##STR9##                                                                     ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________                         Color  Bleach Leakage                                                         70%-20% R                                                                            10%-60% R                                                                            Current                                              HI % R                                                                             LOW % R                                                                             Sec    Sec    A/m.sup.2                                  __________________________________________________________________________    PC (prior art)                                                                          80.6 8.3   4.5    4.8    7.18                                       GBL (prior art)                                                                         78   15.1  7.1    3.4    5.6                                        DMF (prior art)                                                                         81.3 27.2  8.75   1.54   8.40                                       *AN (prior art)                                                                         81   43    1.5    1.5    9.8                                        HPN       81.5 7.5   3.7    5.43   6.76                                       ODPN      80.3 6.1   3.7    >10    2.68                                       ABL       80.1 6.7   3.9    9.4    3.83                                       MS        81.5 7.4   5.0    >10    1.54                                       **MGNT    80.2 7.6   4.5    9.34   3.1                                        PC/ODPN                                                                       75/25     81.4 7.1   4.12   5.51   5.88                                       50/50     80.7 6.3   3.67   7.84   4.79                                       25/75     80.9 6.55  4.03   8.86   3.75                                       PC/ABL                                                                        75/25     80.2 7.4   4.07   5.99   7.16                                       50/50     80.8 6.9   3.89   6.78   5.79                                       25/75     80.5 6.95  4.25   6.86   5.25                                       HPN/ODPN                                                                      75/25     79.4 6.8   3.84   5.63   6.28                                       50/50     79.6 6.55  3.51   7.28   4.79                                       25/75     79.8 6.95  3.78   7.70   4.26                                       HPN/ABL                                                                       75/25     79.1 7.4   4.8    5.97   5.21                                       50/50     79.0 6.7   4.3    7.98   4.65                                       25/75     79   7.0   4.6    8.04   4.3                                        ODPN/ABL                                                                      75/25     78.6 6.9   6.1    12.45  2.84                                       50/50     80.7 6.6   4.54   11.3   3.09                                       25/75     80.2 7.2   5.7    9.44   3.42                                       HPN/MGNT  79.1 7.1   3.52   5.85   4.75                                       50/50                                                                         HPN/MS                                                                        50/50     79.1 7.0   4.31   7.07   4.48                                       75/25     80.5 7.3   4.88   4.12   5.2                                        HPN/ABL/ODPN                                                                            80.6 6.7   3.6    6.6    5.25                                       35/50/15                                                                      __________________________________________________________________________     *Counterion is tetrafluoroborate instead of perchlorate                       **Counterion is hexafluorophosphate                                           AN = Acetonitrile                                                             GBL = Gammabutyrolactone                                                      PC = Propylene Carbonate                                                      HPN = 3Hydroxypropionitrile                                                   ODPN = 3,3'-Oxydipropionitrile                                                ABL = 2acetylbutyrolactone                                                    MS = 3methylsulfolane                                                         MGNT = 2methylglutaronitrile                                                  DMF = Dimethylformamide                                                  

                  TABLE 3                                                         ______________________________________                                        Solubility of UV stabilizers                                                  All data is wt/vol % and at room temperature.                                           PC     ABL    GNT   ODPN  HPN   MGNT                                ______________________________________                                        Tinuvin P ™                                                                          1.7    0.5    0.4   0.7   <0.2  0.9                                 Uvinul 400 ™                                                                         11.9   21.5   12.6  7.6   14.9  11.3                                Cyasorb 24 ™                                                                         13.7   33.5   15.5  15.0  20.9  28.6                                Uvinul    33.6   35.9   35.9  40.2  Immis-                                                                              37.3                                N-539 ™                          cible                                     ______________________________________                                        Tinuvin P ™ = 2-(2H-benzotriazole-2-yl)-4-methyphenyl                      Ciba Geigy, Hawthorne, New York                                               Uvinul 400 ™ = 2,4-dihydroxy-benzophenone                                  BASF Wyandotte Corp., Wyandotte, MI                                           Cyasorb 24 ™ = 2,2'-dihydroxy-4-methoxybenzophenone                        American Cyanamid Company, Wayne, NJ                                          Uvinul N-539 ™ = 2-ethylhexyl-2-cyano-3,3-diphenylacrylate                 (Note: a liquid) BASF Wyandotte, Wyandotte, MI                                PC = Propylene Carbonate                                                      ABL = 2-acetylbutyrolactone                                                   GNT = Glutaronitrile                                                          ODPN = 3,3'-Oxydipropionitrile                                                HPN = 3-Hydroxypropionitrile                                                  MGNT = Methylglutaronitrile                                               

3-Hydroxypropionitrile (HPN)

3-Hydroxypropionitrile has a boiling point of 228° C. and hence canwithstand the high temperatures which can be generated by a mirror orthe like sitting in the sun (Table 1). Similarly, it has a freezingpoint of -46° C. and thus will not freeze in cold winter weather.Electrolytes show excellent solubility, i.e., greater than 0.05 molar.The solutions are clear in an electrochemichromic cell. Further,3-hydroxypropionitrile is a relatively inexpensive solvent, thusaffording economies which are comparable to that obtained by usingpropylene carbonate.

UV stabilizers, like the electrolytes also show excellent solubility inHPN. This makes it possible to enhance UV stability of HPNelectrochemichromic solutions. Both Uvinul 400™ and Cyasorb 24™ showsuperior solubility in HPN as compared to propylene carbonate.

HPN electrochemichromic solutions exhibit a high percentage reflectivityin their bleached condition (81.5%, Table 2). Similarly, theirreflectivity when colored is low, i.e., 7.5%. HPN solutions also tend tocolor fast (3.7 seconds), and they bleach satisfactorily.

One of the most important advantages of HPN solutions over propylenecarbonate solutions is their lower current leakage. The HPN solution ofTable 2 exhibits leakage of 6.76 amperes per square meter versus 7.18amperes per square meter for a comparable propylene carbonate solution(Table 2).

3,3'-Oxydipropionitrile:

3,3'-Oxydipropionitrile (ODPN) has a boiling point of 188° C. even at 16mm Hg (0.021 Atm) and a freezing point of -26° C. (Table 1). This spreadminimizes the probability of either overheating or freezing difficultiesin extreme climate conditions. Electrolytes show excellent to goodsolubility in ODPN (i.e., greater than 0.05 molar). The same is true forUV stabilizers. The ODPN solutions are clear and colorless in anelectrochemichromic cell. Like HPN, ODPN is a relatively economicalcommodity.

Referring to Table 2, it can be seen that ODPN also shows an excellentspread in reflectivity from high to low. It colors rapidly and, while itbleaches somewhat more slowly than HPN solutions, ODPN solutions exhibita very low leakage current at 2.68 amperes per square meter.

Referring to Table 3, it will be seen that Uvinul 400™, Cyasorb 24™, andUvinul N-539™, all well-known and popular UV stabilizers, show superiorsolubility in ODPN as compared to propylene carbonate.

2-acetylbutyrolactone (ABL):

ABL boils at 107° C. even at a vacuum as great as 5 mm Hg (0.0066 Atm)and freezes at lower than -33° C. This is an excellent boilingpoint/freezing point range. The high boiling point and low vaporpressure of this material makes it excellent for vacuum backfilling. ABLalso shows excellent self-screening UV characteristics (discussedBelow).

ABL solutions show an excellent spread between high and low reflectivity(80.1 to 6.7 as reported in Table 2). They color rapidly (3.9 seconds)and bleach fairly rapidly at 9.4 seconds. The current leakage is alsodesirably low at 3.83 amps per square meter.

Referring to Table 3, it will be seen that Uvinul 400™, Cyasorb 24™ andUvinul N-539™ all show superior solubility in ABL as compared topropylene carbonate.

2-methylglutaronitrile (MGNT):

2-methylglutaronitrile also has an extremely high boiling point. Even at10 mm of mercury (0.013 atmospheres) it boils at 125° to 130° C. Itsextremely low freezing point of less than -33° C. gives this solventexcellent environmental range. The high boiling point and low vaporpressure also make it excellent for facilitating vacuum backfilling.

Leakage current is extremely low for this solvent at 3.1 amperes persquare meter. Even when used 50:50 with HPN in the solutions of Table 2,it has a leakage current of only 4.75 amps per square meter. Similarly,the percent reflectance range for the 50:50 solvent combination isexcellent, as are the color and bleach times (Table 2).

3-methylsulfolane (MS):

3-methylsulfolane boils at 276° C. and freezes at -10° C. (Table 1).This gives this solvent and its solutions excellent temperaturestability. The high boiling point also ensures a relatively low vaporpressure, making this an excellent solvent for use in vacuum backfillingapplications.

Electrochemichromic solutions based on this solvent exhibit the best lowleakage current at 1.54 amperes per square meter. Even in a 50:50 mixwith HPN, the Table 2 solutions show a leakage current of 4.48 amperesper square meter. At 75% HPN/25% MS, the leakage current is still only5.2 amps per square meter.

The reflectance range from high to low is very acceptable for thissolvent (Table 2). Similarly, the time to color is fast and althoughbleach is somewhat slow, the bleach response time would be veryacceptable in window applications, particularly large area windows whichbenefit from the extremely low leakage current of MS (Table 2).

Solvent Mixtures:

The Table 2 data demonstrates that the solvents of the present inventionalso work well in combination with each other and in combination withprior art solvents. Thus, the inherent properties of prior art solventsare enhanced when they are combined with a solvent of the presentinvention wherein the combination comprises at least about 25% by volumeof the solvent of the present invention.

The Table 2 results for propylene carbonate alone should be compared tothe results achieved for propylene carbonate combined with 25%, 50% and75% by volume ODPN and ABL. In each case, the combination of propylenecarbonate with one of the solvents of the present invention shows a moredesirable leakage current over propylene carbonate per se, without asignificant degradation in clear to colored spread, time to color ortime to bleach.

Table 2 also discloses the combination of HPN and ODPN. It can be seenthat the properties of this combined solution at 75:25, 50:50 and 25:75are superior in terms of low leakage current to those of HPN alone. Yet,the colored to uncolored light transmission spread is still exceptional.The time to color and the time to bleach are similarly superior to theperformance achieved by ODPN alone.

Table 2 further discloses combinations of various other solvents of thepresent invention. Thus, beneficial combinations are seen involving HPNand ABL, ODPN and ABL, HPN and MGNT and HPN and MS. In all cases, theresults in terms of high and low reflectance, time to color, time tobleach and leakage current are exceptional.

Finally, Table 2 illustrates that even more complex mixtures of thesolvents of the present invention yield complementary and desirableresults. Thus, Table 2 includes data obtained using electrochemichromicsolution in 35% HPN, 50% ABL and 15% ODPN. As with other combinationsshown in Table 2 involving the present invention, the results areexceptional.

Thus by using blends of the solvents of the present invention eitherwith each other or with prior art solvents, one can obtain a combinedsolvent with desirable attributes of both. The faster times to color andbleach of a prior art solvent such as propylene carbonate can becombined with the lower leakage current of solvents such as MS, ODPN orABL by incorporating at least about 25% of a solvent in accordance withthe present invention.

Prior Art Solvents:

Referring to the prior art solvents, it can be seen that except forgammabutyrolactone (GBL), they all have a relatively high leakagecurrent, i.e., in excess of 7.18 amps per square meter. While GBL has arelatively low leakage current, its low end light transmittance, i.e.,transmittance when colored, is relatively high. In theelectrochemichromic mirror as described, it exhibits 15.1% reflectance,as distinguished from less than 10% reflectance for electrochemichromicmirrors made using solvents of the present invention. GBL also has arelatively high vapor pressure, i.e., 3.2 mm Hg at 25° C., making vacuumbackfilling difficult. Dimethylformamide (DMF) and acetonitrile (AN)performed even worse in terms of percent reflectivity when colored(i.e., 27.2% and 43% respectively).

There is one solvent suggested in the prior art which does performcomparably to the solvents of the present invention. U.S. Pat. No.3,806,229 to Schoot suggests the use of glutarodinitrile(glutaronitrile) as a solvent along with acetonitrile, propionitrile,benzonitrile, propylene carbonate, nitromethane and acetic acidanhydride. Glutaronitrile does show reduced current leakage andself-screening UV characteristics comparable to the solvents claimedherein. However, these favorable properties are totally unappreciated bySchoot and are not disclosed in U.S. Pat. No. 3,806,229. Accordingly,the use of glutaronitrile to achieve these desirable properties will bethe subject of a copending United States patent application.

Self-Shielding UV Stabilization:

The solar spectrum in the ultraviolet (UV) region incident at a desertlocation such as Tucson, Ariz., is shown in FIG. 1. The Y ordinate isthe solar energy expressed in microwatts/cm² /100 A° band. This solarspectrum must typically pass through an ITO coated glass front piece toirradiate the solution in an electrochemichromic cell 1 as shown in FIG.2. The transmission of ITO coated glass (0.063" sodalime coated to halfwavelength thickness with 1500 A° ITO) is shown in FIG. 3. Thus thesolar energy spectrum transmitted into the electrochemichromic fluid isthe convolution of FIG. 1 with FIG. 3. This is shown in FIG. 4. The ITOcoated glass passes about 55% of the incoming UV solar energy in the 250to 350 nm region. Thus a substantial portion of the solar UV isunattenuated by the ITO coated glass front piece. This UV radiationpasses into the electrochemichromic fluid where it irradiates theelectrochemichromic species dissolved therein.

As shown in FIG. 5, the cathodically coloring species most commonly usedin prior art literature such as methylviologen (MV), ethylviologen (EV),benzylviologen (BV), and heptylviologen (IV) have an absorption peakbelow 295 nm and thus should be largely nonabsorbing to the solar UVtransmitted into the electrochemichromic cell. However, as shown in FIG.6, anodic compounds, such as dimethyldihydrophenazine (DMPA),diethyldihydrophenazine (DEPA), tetramethylphenylenediamine (TMPD),tetramethylbenzidine (TMBZ) and tetrathiafulvalene (TTF) havesubstantial UV absorbance in the 250 to 350 nm region. For example, DMPAin 0.0002M solution in acetonitrile (AN) and in a 1 mm pathlength quartzcell absorbs about 22% of the UV solar spectrum passed by ITO coatedglass in the 250 to 350 nm region.

Thus, it is desirable to shield the electrochemichromic compounds fromUV irradiation in this region. One aspect of the present inventioninvolves the use of a solvent which self-screens solutes dissolvedtherein from the detrimental effects of UV radiation impinging thereon.Specifically., the solvent in a one millimeter path length must transmitno more than about 30% of the solar radiation in the 250 to 350 nm rangepassed through a typical transparent ITO coated glass substrate. Such asubstrate would typically be soda lime glass of approximately 0.063 inchthickness, coated with a half wave (1500° A) ITO transparent conductivelayer. One of the solvents of the present invention, in addition toacting as solvent for electrochemichromic compounds such that theresulting electrochemichromic solution cycles well from a hightransmitting state to a low transmitting state, has the additionalbeneficial property of self-absorbing substantial proportions of the UVsolar insolation in the 250 to 350 nm region. FIG. 7 shows thetransmission of such solvent, acetylbutyrolactone (ABL). The spectrum istaken in a 1 mm pathlength quartz cell. This 1 mm cell filled with ABLtransmits from 250 nm to 350 nm only about 10% of the solar radiationpassed by half wave (1500° A) ITO coated standard soda lime glass of athickness of about 0.063 inches. This data can be compared to FIG. 8,which shows the UV transmission, in a 1 mm pathlength cell, for varioussolvents proposed in prior art for use in electrochemichromic solutions.Note that these are mostly completely transmitting in the ultravioletregion in that they absorb very little between 250 nm to 350 nm. Forexample, propylene carbonate in a 1 mm cell transmits about 87% of thesolar energy passed through ITO coated glass between 250 nm and 350 nm;dimethylformanide (DMF) about 89%, acetonitrile (AN) about 100%; andmethyethylketone (MEK) about 93%. Thus, ABL helps extend the UV lifetimeof electrochemichromic solutions by shielding the UV fragileelectrochemichromic compounds that are solutes in the UV self-screeningsolvent.

Self-screening by the solvent is more effective than screening with a UVinhibiting solute because the solvent is by far the majority componentin the solution. For example in a 0.025M solution in ABL, the molarityof the solute is 0.025M while the molarity of the solvent is 9.29M sothat there are roughly 370 solvent molecules for every solute moleculein solution. Thus, the probability is greatest that an incoming UVphoton may impinge and be absorbed by a solvent molecule (whichtypically is UV resilient), rather than impinge and be absorbed by asolute molecule (which, in the case of electrochemichromic species, isusually UV fragile and degraded by UV irradiation).

Although addition of UV stabilizers such as benzotriazoles,benzophenones, or hindered amine complexes, as known in prior art, canhelp increase solution stability to UV radiation, there are limitationsand disadvantages to addition of UV stabilizers. Because they are heldin solutions of low to moderate viscosity, both the UV stabilizer andthe electrochemichromic solution species it is intended to stabilize arefree to randomly move about in the solution. Thus, an incoming photon ofUV radiation may impinge and thus degrade an electrochemichromic solutespecies rather than a UV absorber in solution.

Also, solvent solubility places limits on the amount of UV stabilizerthat can be added. Since UV stabilizers typically have molecular weightsin the 250 to 400 range, the molarity of UV stabilizer in solution istypically around 0.2M or less if the stabilizer concentration is 5%wt/volume. Thus UV stabilizers outnumber solute by about 10 to 1 but thesolvent outnumbers solute by about 350 to 1 and thus the inherentself-screening achievable by the solvent ABL, working in conjunctionwith dissolved stabilizer, can help enhance the UV stability ofelectrochemichromic devices that use these types of UV self-screeningsolvents.

ADDITIONAL EXAMPLES

The following examples further illustrate the important and unexpectedadvantages of the solutions of the present invention over the prior art:

Example 1 (Prior Art DMF)

A self-erasing electrochemichromic cell solution was prepared based onthe prior art of Shelepin, as taught in Elektrokhimya, 13(3), 404-408(March 1977). This consisted of:

0.05M Methylviologen perchlorate 0.05M5,10-dihydro-5,10-dimethylphenazine 10% weight/volumepolymethylmethacrylate (90,000 average molecular weight) dissolved indimethylformamide (DMF).

In addition, 0.05M tetraethylammonium perchlorate was added as inertcurrent carrying electrolyte as taught in Manos U.S. Pat. No. 3,451,741(Jun. 24, 1969).

The solution was prepared under oxygen-free conditions and anhydrousargon gas was bubbled through the solution to further deoxygenate thesolution. A roughly 23 cm×5.6 cm rectangular cell was fabricatedaccording to the construction shown in FIG. 2. The cell cavity wasformed by silk-screening a roughly 2 mm×150 micron epoxy seal around theedge perimeter of one of the ITO transparent conductive coated glasssubstrates shown in FIG. 2. Sheet resistance for the ITO transparentconducting, coating used on both substrates was about 15 ohms/square.Prior to its silk-screening, glass beads of nominal diameter 150 micronswere mixed with the epoxy. Before curing of the epoxy, the second ITOcoated glass substrate was contacted to the epoxy seal and the nowlaminated construction was baked in an oven to cure the epoxy. A smallgap of approximately 2 mm×1 mm×150 micron dimension had been allowed inthe epoxy seal so that, upon lamination, a small fill hole was availableclose to one corner of the seal through which fluid could flow duringthe vacuum backfilling process. Attempts at room temperature to vacuumbackfill this solution failed. When vacuum was applied, the DMF-basedsolution boiled and could not be filled into the cell cavity.

With this prior art solution, two fill holes were drilled through theface of one of the ITO coated glass substrates so that fluid could befilled into the cell cavity using suction at one hole to pull solutionfrom a syringe tightly held to the other hole. Each hole was ofapproximately 1 mm diameter. For this construction, no fill hole wasallowed in the epoxy seal.

After the DMF-based electrochemichromic solution described above wasfilled into the cell cavity using suction pull through from a syringe,the holes drilled through the glass substrate were plugged with epoxy.Using this nonvacuum backfilling technique, the prior art DMF solutioncould be successfully filled into the cell cavity. This fillingtechnique, although practical at a laboratory or prototype level, hasdisadvantages for commercial devices which include difficulties insecurely plugging the relatively large fill holes drilled through theglass substrate.

Consistent with prior art teachings, electrochemichromic windows andmirrors, produced as described above and using the DMF-based formulationfrom prior art, were found to have the variable transmission (orvariable reflection in the case of mirrors), cycle lifetime andcoloration efficiency required to render single-compartment,self-erasing, solution-phase electrochemichromic devices commerciallypractical.

For example, a roughly 129 cm² window was constructed of dimension 23cm×5.6 cm×150 microns cell thickness. When filled with the prior artDMF-based formulation, and where a silver mirror reflector was placedbehind this window, the reflectance from the mirror, which initially was81.3% R, dimmed rapidly to about 27.2% R as measured at the center ofthe rectangular device.

To dim the mirror reflectance, a potential of 1 volt was applied to busbars that ran lengthwise along the outer perimeter of the ITO coatedtransparent substrates that sandwiched the DMF-based electrochemichromicsolution. Upon removing the applied potential, the electrochemichromicsolution self-erased back to a clear state so that the mirrorreflectance returned to 81.3% R. Alternatively, the cell could be morerapidly bleached by shorting the electrodes. Cycle lifetime wasestablished by applying 1 volt potential across the electrochemichromicsolution for 30 seconds to dim the transmission, followed by shortingthe cell electrodes for 30 seconds to bleach back to the clear state.This color for 30 seconds followed by bleach for 30 seconds cycle wascontinued for over 10,000 cycles. Coloring efficiency was maintained;the clear state reflectivity remained high while the cell continued todim in its center to about 27% R.

However, there are three significant disadvantages to using this priorart DMF-based formulation for commercial applications. The large leakagecurrent which was in excess of 8 A/m² would lead to undesirable powerdrain in practical devices and particularly in large area window ormirror devices. Also, although coloration was efficient as seen by thedeep coloration close to the bus bars, coloration was very nonuniform,even for this relatively small window of 129 cm² area. Also, althoughthickener was added as suggested by prior art to limit segregation,segregation was nevertheless present even after relatively modestprolonged coloration. For these reasons, coupled with the impracticalityof using vacuum backfilling, this prior art DMF-based solution was seento be inferior to mirror and window devices made possible by thesolutions of this present invention.

Example 2 (HPN Solution)

A self-erasing electrochemichromic solution was formed of:

0.025M Methylviologen perchlorate 0.025M5,10-dihydro-5,10-dimethylphenazine 0.025M Tetraethylammoniumperchlorate 5% weight/volume 2,4-dihydroxy-benzophenone (UV stabilizer"Uvinul 400™) dissolved in hydroxyproprionitrile (HPN).

This solution was filled using vacuum backfilling into an interiorrearview mirror shaped window constructed as per FIG. 2 of length about24 cm and width about 5 cm, and of cell area about 110 cm². Theinterpane gap was 150 microns. Glass coated with ITO of sheet resistance15 ohms/square and greater than 85% visible transmittance was used. Asilver mirror reflector was placed behind the window. Without anyapplied voltage, the cell was clear and colorless and the reflectancefrom the mirror was about 81.5% R. When 1 volt potential was appliedacross the cell, reflectance of the mirror was reduced to 7.5% R, asmeasured at the center of the window device. Color transition time from70% R to 20% R was 3.7 seconds. When the electrodes were shorted, bleachtime from 10% R to 60% R was 5.4 seconds. Coloration was both efficientand satisfactorily uniform. Leakage current was about 6.8 A/m².

After prolonged colored for 30 minutes, segregation performance asevidenced by a blue band adjacent to the cathodically powered bus barand a yellow/brown band adjacent to the anodically colored bus bar wassmall. Segregation performance and uniformity were greatly improved overthat seen in Example 1, even though no additional thickening agents suchas are taught to be necessary in Shelepin, supra Example 1, and in BykerEuropean Patent Publication 240,226 were used. Nor was the use of a highconcentration of current-carrying salt necessary, such as is taught tobe necessary for commercial practicality of the solutions taught inByker European Patent Publication 240,226.

The HPN-based formulation has the coloring efficiency and uniformityrequired to be commercially practical and it showed unexpectedlyexcellent cycle lifetime. Cells fabricated as described in this examplehave been cycled in excess of 100,000 cycles without any significantdeterioration in the performance described above. Each cycle consistedof 30 seconds color at 1 V applied, and 30 seconds bleach at 0 Vapplied, i.e., with the electrodes shorted. The cell is undamaged whensubjected to prolonged coloration. Performance is maintained afterbaking at 85° C. for two weeks. Cells are not damaged by prolongedstorage at low temperatures of -20° C. or lower. The formulationdescribed here is sufficiently UV stable, at least for use in theinterior cabin of an automobile.

Example 3 (ABL Solution)

A self-erasing electrochemichromic solution was formulated as describedin Example 2, but using 2-acetylbutyrolactone (ABL) as the solvent. Whenfilled into a cell as described in Example 2, and with a silver mirrorreflector placed behind the window, mirror reflectance was 80.1% R inthe clear state, which dimmed to 6.7% R when 1 volt was applied. Colortime to dim from 70% R to 20% R was 3.9 seconds. Leakage current wasabout 3.8 A/m². Bleach time from 10% R to 60% R was 9.4 seconds and thuswas adequate for an automotive rearview mirror application, and veryacceptable in a window application. Coloring efficiency and coloringuniformity were excellent, as was segregation performance which wasexceptionally good. Cells formulated according to this example haveexcellent cycle lifetime with in excess of 86,000 30 second color; 30second bleach cycles tested without any significant deterioration inperformance. The high temperature, low temperature, and UV performancefor this solution are similar to that reported above in Example 2.

The low leakage current discovered in ABL solutions makes this formulaparticularly well suited for use in large area windows and mirrors wherevoltage drops across transparent conductors due to excessive leakagecurrents leading to undesirable and commercially impractical nonuniformcoloration along with excessive segregation.

Example 4 (50:50 HPN/ODPN)

A self-erasing electrochemichromic solution was formulated as describedin Example 2, but using a mixture of 50% by volume HPN and 50% by volumeODPN as solvent. Using this solvent mixture, this solution was filledinto a cell as described in Example 2. With a silver mirror reflectorplaced behind the window so formed, the initial reflectance of 79.6% Rdimmed to about 6.6% R when 1 volt was applied across the cell. Colortime from 70% R to 20% R was 3.5 seconds. Moreover, the bleach time from10% R to 60% R was about 7.3 seconds which is intermediate between the5.4 seconds found using pure HPN and the greater than 10 seconds foundusing pure ODPN. Leakage current was about 4.8 A/m². Thus, a solventthat shows the surprisingly and unexpectedly low leakage current foundwith ODPN can be proportionately combined with a higher leakage currentsolvent such as HPN of this invention or propylene carbonate from priorart, so that the leakage current of the resultant mixture can becustomized to suit the commercially desirable performance of thespecific product involved. Since a low leakage current solvent givesbetter color uniformity and lower segregation, but typically also leadsto a slower bleach response, solvents such as HPN and ODPN can be mixedto yield the optimum compromise between color uniformity and segregationperformance and bleach response time.

Electrochemichromic solutions, as described in this example, thatutilize a 50:50 mixture of HPN and ODPN have the commercially requiredhigh temperature, low temperature, and UV performance required forcommercial use. Their cycle lifetime is excellent. They show nosignificant change in performance after 48,000 cycles at +25° C., 6,000cycles at 240° C., and 6,000 cycles at -0° C., each cycle consisting ofcoloring at 1 volt applied potential for 15 seconds, and 0 volt appliedpotential for 15 seconds.

Example 5 (ODPN, MS. MGNT)

Solutions were formulated, and devices fabricated and tested asdescribed in Example 2, but using oxydipropionitrile (ODPN) as thesolvent. Excellent cycle lifetime, coloring efficiency, coloringuniformity, high temperature, low temperature, and ultravioletperformance was achieved. Likewise, excellent performance was recordedwith the other solvents novel to this invention, 2-methylglutaronitrileand 3-methylsulfolane, whether used as pure solvents or as mixturesamong themselves, or with HPN and ABL, or with prior art solvents suchas propylene carbonate.

Example 6 (Prior Art PC)

To illustrate the benefit of customizing leakage current so thatcoloration uniformity, low segregation, and bleach response isoptimized, square windows of dimension 14 cm×14 cm×150 micron cellthickness were fabricated according to the construction schematicallyshown in FIG. 2. A window so constructed that used ITO of sheetresistance about 25 ohms/square was filled with a solution comprising:

0.025M Methylviologen perchlorate 0.025M5,10-dihydro-5,10-dimethylphenazine 0.025M Tetraethylammoniumperchlorate in pure propylene carbonate.

Bus bars were attached around the perimeter edges and a potential of 1volt was applied. At rest potential, the electrochemichromic window soformed was about 86% transmitting. This window colored deeplyimmediately adjacent to the bus bars, but did not color in the center,such that such a window would be commercially unacceptable. Also, whenthis window was prolonged colored at 1 volt for minutes, verysignificant segregation of the reduced cathodic species to the one busbar and the oxidized anodic species to the other bus bar was seen.

When another cell was filled with this solution, but this time using ITOof sheet resistance about 15 ohms/square as the transparent conductorsthat sandwiched the electrochemichromic solution, the center of thewindow still failed to dim appreciably when 1 volt was applied. Thecenter dimmed only to about 0.40% T (although, as can be expected giventhe lower sheet resistance of ITO used, the colored region closer to thebus bars increased somewhat in area, while the central nondimmed areaproportionally decreased in size). Segregation after prolongedcoloration at 1 volt for 15 minutes was improved over what was seen asdescribed above when this solution had been used in a cell that utilized25 ohms/square ITO but was nevertheless still significant.

Example 7 (ODPN in 15 ohms/Square ITO Cell)

By contrast, when a similar cell to that described in Example 6 wasfilled with equivalent electrochemichromic solution, but this time usingthe ODPN of this invention and using 15 ohms/square ITO, the centralregion dimmed appreciably (down to 14% T) and uniformly such that thewindow would be commercially practical. Segregation after prolongedcoloration at 1 volt for 15 minutes for the ODPN-based solution was onlyslight to negligible and was demonstrably superior to the windowsdescribed above that utilized propylene carbonate based solutions.Applications such as large area architectural and automotive windows,office partition panels, and large area mirrors become practical usingthe low leakage current solvents, such as ODPN, of this invention.

Example 8 (Ethylviologen in HPN/ODPN)

A self-erasing, electrochemichromic solution was formulated comprising:

0.025M ethylviologen perchlorate 0.025M5,10-dihydro-5,10-dimethylphenazine 0.025M Tetraethylammoniumperchlorate 5% weight/volume 2,4-dihydroxy-benzophenone (UV stabilizerUvinul 400™ dissolved in a 50:50 hydroxypropionitrile(HPN)/oxydipropionitrile (ODPN) mixture.

When filled into a 24 cm×5 cm×150 micron cell, as described in Example2, a silver mirror reflector placed behind the window so formed had areflectivity of 80.1% R which dimmed to 6.7% R when 1 volt was appliedacross the ITO transparent conductors (of 15 ohms/square sheetresistance) used. Coloration was rapid (3.4 sec) and bleach response wassatisfactory. Excellent coloration efficiency and coloration uniformitywere maintained after cycle lifetime testing in excess of 60,000 cycles;each cycle consisting of 1 volt applied for 30 seconds, followed by OVapplied for 30 seconds.

Example 9 (TMPD Anodic Material In HPN/ODPN)

A self-erasing electrochemichromic solution was formulated comprising:

0.035M methylviologen perchlorate 0.035M N,N,N¹,N¹-tetramethyl-1,4-phenylenediamine 0.035M Tetraethylammonium perchlorate5% weight/volume Uvinul-400 dissolved in a 50:50 mixture ofhydroxypropionitrile (HPN) and oxydipropionitrile (ODPN).

When an electrochemichromic window was assembled, as essentiallydescribed in Example 2, the clear, zero potential transmission was about81% T and this decreased to about 11% T when a potential of 0.6 V wasapplied across the cell. Coloration from 70% to 20% T took about 7.3seconds, bleach from 11% T to 60% T took about 7 seconds. Upon fullybleaching at zero potential, the transmission returned to the clear,high transmitting state. When repetitively cycled at 0.6 V for 30seconds followed by 0 V for 30 seconds, the performance cited above wasessentially maintained.

Example 10 (TTF Anodic Material In HPN/ODPN)

A self-erasing electrochemichromic solution was formulated comprising:

0.025M methylviologen perchlorate 0.025M Tetrathiafulvalene 0.025MTetraethylammonium perchlorate dissolved in a 50:50 mixture ofhydroxypropionitrile (HPN) and oxydipropionitrile (ODPN).

When an electrochemichromic window was assembled, as essentiallydescribed in Example 2, the clear, zero potential transmission was about80.3% T. When a potential of 1.0 V was applied across the cell and witha silver mirror reflector behind the window, the mirror reflectancedimmed from its initial high reflectance state of 80.3% R down to a lowreflectance state of 5.8% R. Coloration time from 70% to 20% R was 3.2seconds; bleach from 10% R to 60% R took 7.1 seconds.

Example 11 (HPN/ABL-Variable Transmission)

A window was formed as described in Example 2 consisting of:

0.025M Methylviologen perchlorate 0.025M5,10-dihydro-5,10-dimethylphenazine 0.025M Tetraethylammoniumperchlorate in a 50:50 mixture of hydroxypropionitrile (HPN) andacetylbutyrolactone (ABL).

This cell was powered at various voltages from 0 V to 1.2 V. Thetransmission at, the center of this window, at various applied voltages,is shown in Table 4.

                  TABLE 4                                                         ______________________________________                                        Applied Voltage Volts                                                                          % Transmission                                               ______________________________________                                          0 V            78.9                                                         0.2 V            78.9                                                         0.6 V            46.2                                                         0.7 V            31.4                                                         0.8 V            21.4                                                         0.9 V            14.8                                                         1.0 V            11.3                                                         1.1 V            8.6                                                          1.2 V            7.3                                                          ______________________________________                                    

As can be seen from the Table, % transmission can be varied between78.9% T and 7.3% T by appropriately selecting the applied voltage. Thisability to operate as a grey scale was found in all the novel solventsand solvent mixtures disclosed in this invention, and is consistent withprior art teachings and with the intrinsic properties of theelectrochemichromic species themselves.

Example 12 (HPN)

A self-erasing electrochemichromic solution was formed of:

0.035M Methylviologen hexafluorophosphate 0.035M5,10-dihydro-5,10-dimethylphenazine 0.035M Tetrabutylammoniumhexafluorophosphate dissolved in hydroxypropionitrile (HPN).

This solution was filled using vacuum backfilling into an interiorrearview mirror shaped window constructed as per FIG. 2 of length 24 cmand width 5 cm, and of cell area 110 cm². The interpane gap was 150microns. Glass coated with ITO of sheet resistance 15 ohms/square andgreater than 85% visible transmittance was used. A silver mirrorreflector was placed behind the window. Without any applied voltage, thecell was clear and the reflectance from the mirror was about 78.8% R.When 1 volt potential was applied across the cell, reflectance of themirror was reduced to 5.8% R, as measured at the center of the windowdevice. Color transition time from 70% R to 20% R was 3.8 seconds. Whenthe electrodes were shorted, bleach time from 10% R to 60% R was 5.6seconds. Coloration was both efficient and satisfactorily uniform. TheHPN-based formulation has the coloring efficiency and uniformityrequired to be commercially practical and it showed unexpectedlyexcellent cycle lifetime. Cells fabricated as described in this examplewere cycled without any significant deterioration in the performancedescribed above. Each cycle consisted of 30 seconds color at 1 Vapplied, and 30 seconds bleach at 0 V applied, i.e., with the electrodesshorted. The cell is undamaged when subjected to prolonged coloration.Performance is maintained after baking at 85° C. for two weeks. Cellsare not damaged by prolonged storage at low temperatures of -20° C. orlower. The formulation described here is sufficiently UV stable, atleast for use in the interior cabin of an automobile.

Example 13 (Commercial Cell Comparison)

The practical benefit of these concepts can be illustrated by comparingthe UV stability of a rearview mirror fabricated using the concepts ofthis invention to the UV-stability of a commercially availableelectrochemichromic rearview mirror. The particular commerciallyavailable electrochemichromic mirror tested was an interior rearviewmirror purchased from General Motors spare parts around July 1989 and itwas marked A105/1.16 on its rear surface. Analysis of these commerciallyavailable mirrors shows that their solutions contain benzylviologen (BY)and 5,10-dihydro-5,10-dimethylphenazine (DMPA ) in propylene carbonateand also contain a benzothriazole UV stabilizer. Our electrochemichromicrearview mirrors consisted of: 0.025M methylviologen perchlorate; 0.025Mdimethydihydrophenazine; 0.025M tetraethylammonium perchlorate; 5% byweight 2,2'-dihydroxy-4-methoxybenzophenone (Cyasorb 24™) as UVstabilizer all dissolved in 50% HPN:50% ABL and in 50% ABL:50% ODPN. Allthree mirrors were exposed for 89 hours to a xenon arc lamp and wereirradiated with UV radiation that closely simulated the solar UVintensity. The integrated intensity from 250 nm to 400 nm was around 70watts/M². Results were:

    ______________________________________                                                               Color      Bleach                                               HI    LOW     70%-20% R  10%-60% R                                            % R   % R     Sec        Sec                                         ______________________________________                                        Initially                                                                     Commercially                                                                             79.9    6.6     2.95     4.22                                      Available ECCM                                                                HPN/ABL    80.9    8.1     2.06     8.16                                      50/50                                                                         ABL/ODPN   81.7    8.9     2.39     12.22                                     50/50                                                                         89 Hour UV                                                                    Exposure                                                                      Commercially                                                                             59.6    7.6     2.72*    6.05                                      Available ECCM                                                                HPN/ABL    80.6    6.6     1.66     7.64                                      50/50                                                                         ABL/ODPN   79.6    9.4     2.55     11.62                                     50/50                                                                         ______________________________________                                         *59.6% R to 20% R                                                        

The formulations in accordance with the present invention performedremarkably better than the commercially available formulation in thatthey maintained their high reflectance state of about 80% R, whereas thecommercial product dropped its zero potential reflection to only about60% R, principally due to its yellowing.

Of course, it is understood that the above is merely a preferredembodiment of the invention and that various changes and alterations canbe made without departing from the spirit and broader aspects thereof asset forth in the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An electrochemichromicsunroof for a vehicle comprising:spaced plates, each having an inwardlyfacing conductive surface; an electrochemichromic solution in said cell,between said spaced plates, said solution comprising: solvent; a redoxchemical pair in solution in said solvent which colors in the presenceof an applied voltage and which bleaches to a colorless condition in theabsence of an applied voltage; said solvent including at least 25 % byvolume of a solvent selected from the group consistingof:3-hydroxypropionitrile (HPN), 3,3'-oxydipropionitrile (ODPN),2-acetylbutyrolactone (ABL), 2-methylglutaronitrile (MGNT), and mixturesthereof.
 2. The electrochemichromic sunroof of claim 1 in which saidelectrochemichromic solution additionally includes an electrolyte insolution in said solvent.
 3. The electrochemichromic sunroof of claim 1in which said solvent comprises: 3-hydroxypropionitrile and2-methylglutaronitrile in a ratio by volume of from about 75:25 to about25:75.
 4. The electrochemichromic sunroof of claim 1 in which saidsolvent comprises: 3-hydroxypropionitrile and 3-methylsulfolane in aratio by volume of from about 75:25 to about 25:75.
 5. Theelectrochemichromic sunroof of claim 1 in which said solvent comprises:3-hydroxypropionitrile and 3,3'-oxydipropionitrile in a ratio by volumeof from about 75:25 to about 25:75.
 6. The electrochemichromic sunroofof claim 1 in which said solvent comprises: 3-hydroxypropionitrile and2-acetylbutyrolactone in a ratio by volume of from about 75:25 to about25:75.
 7. The electrochemichromic sunroof of claim 1 in which saidsolvent comprises: 2-acetylbutyrolactone and 3,3'-oxydipropionitrile ina ratio by volume of from about 75:25 to about 25:75.
 8. Theelectrochemichromic sunroof of claim 1 in which said solvent comprises:propylene carbonate and 3,3'-oxydipropionitrile in a ratio by volume offrom about 75:25 to about 25:75.
 9. The electrochemichromic sunroof ofclaim 1 in which said solvent comprises: propylene carbonate and2-acetylbutyrolactone in a ratio by volume of from about 75:25 to about25:75.